This invention relates to a method for coating a difficult to solder material which makes up an electronic part with solder, and to a coated material which is obtained in this manner and to an electronic part, and particularly to a portion to be soldered of an electronic part.
At the time of joining lead frames for electronic parts and lids and the like of packaged electronic parts, soldering is carried out. At this time, solder is usually supplied from the outside and soldering is carried out.
In the past, metal materials which have been used in such electronic parts have been metal materials such as copper and copper alloys to which solder readily adheres. However, copper and copper alloys are expensive, and their mechanical strength is not sufficient, so recently, metal materials other than copper and copper alloys have come to be used. Among such metal materials are Kovar (Fe-29Ni-17Co), Alloy 42 (Fe-42Ni), and the like. These metal materials are less expensive than copper and copper alloys and also have superior mechanical properties, but they are much inferior with respect to solderability.
Recently, in the soldering of electronic parts, with the intention of rationalizing soldering, soldering technology has been developed in which solder is not supplied from the exterior each time, but a somewhat excess amount of solder is adhered in advance to a portion to be soldered, and soldering is carried out only with the excess amount of solder which is adhered in advance. Below, this technology will be referred to as the xe2x80x9cpreplating methodxe2x80x9d.
Electronic parts to which solder is adhered in advance in this manner and in particular portions thereof to be soldered (such as lids and lead frames) include materials in which plate-shaped solder and a difficult to solder material are attached to each other under pressure by rolling (referred to below as solder clad materials), materials to which solder is adhered by immersing a difficult to solder material in molten solder (referred to below as solder coated materials), and the like, which are formed into products by stamping with a press.
In solder clad materials, complete metallurgical bonding does not occur between the plate-shaped solder and the difficult to solder material, so when a solder clad material undergoes stamping with a press to form a part having a desired shape such as a lead frame or a lid, there are cases in which the plate-shaped solder and the difficult to solder material peel from each other, and the difficult to solder material becomes unable to be wet by solder in some locations at the time of soldering.
Furthermore, even with solder coating of a difficult to solder material, it is desirable to adhere an excess amount of solder, but such a sufficient amount of solder can not be provided in advance with certainty. In addition, in this case, there is a problem if too large an amount of solder is adhered.
For example, in the case of a lid of a semiconductor package, if too large an amount of solder is supplied, the excess solder drips off, and it can be the cause of a malfunction of the electronic device housed in the interior. Furthermore, in the case of a lead frame, if there is a large amount of solder, short circuits with adjoining lead wires occur, and in the case of a small electronic part which is sealed after soldering, if there is a large amount of solder, its volume becomes large after sealing, so it is necessary to provide a necessary and sufficient amount of solder in advance with certainty.
Herebelow, this invention will be explained using a solder coated material for a lid as an example.
As already described, difficult to solder metal materials (such as Kovar and Alloy 42, which are referred to below as difficult to solder materials) have recently come to be used even as materials for lids, and it has been found that it is not possible to provide in advance and with certainty a necessary and sufficient amount of solder with respect thereto.
When a solder coated material is obtained by immersing a difficult to solder material in molten solder, it is conceivable to use a highly active flux containing a large amount of a halogen component with respect to a difficult to solder material having poor solderability. However, when performing soldering with a highly active flux, if even a small amount of flux residue remains after soldering, it corrodes the solder and the difficult to solder material, and a corrosion product is produced, which ends up worsening solderability. For this reason, in a solder coating using a flux, after solder is adhered, it was always necessary to perform complete cleaning. A continuous material is advantageous from the standpoint of productivity when manufacturing a solder coated material, and in general a continuous material is used, but in order to perform complete cleaning, it is necessary to use a large cleaning tank and a large amount of cleaning water, and large expenses are necessary for the initial cost of the cleaning tank and the running costs of the water which is used.
From in the past, it has been known that if ultrasonic waves are applied to molten solder and soldering is carried out therein, soldering can be carried out even without a flux. In soldering with ultrasonic waves, the ultrasonic waves peel off oxides and grime adhering to portions to be soldered by means of strong vibrations, and by exposing a clean metal surface, solder can be completely adhered from a metallurgical standpoint.
When ultrasonic waves were employed when applying molten solder plating to a difficult to solder material such as an iron-nickel compound and plating was attempted, in an experiment performed by the present inventors, adequate soldering with respect to a difficult to solder material was not accomplished no matter how strong the ultrasonic waves which were applied.
Accordingly, an object of the present invention is to provide a molten solder plating method which is effective and highly economical with respect to difficult to solder materials.
Another object of the present invention is to provide a solder coated material for use in a portion to be soldered of an electronic part such as a lead frame or a lid for a package and a manufacturing method therefor.
Yet another object of the present invention is to provide a portion to be soldered of an electronic part such as a lead frame or a lid for a package.
A material used in the present invention is a difficult to solder material such as an iron-nickel alloy. It is known that even with such a material, plating can be easily applied by electroplating. However, it is not possible to use a large amount of solder when performing electroplating with solder, and a solder plated material which is obtained by electroplating cannot be used in the preplating method.
The present inventors discovered that if a layer of solder plating on a portion to be soldered of an electronic part has a thickness of 10-50 micrometers, the preplating method can be effectively carried out. Furthermore, they found that even though the amount of solder can not be made this large by electroplating with solder, a difficult to solder material can be easily plated by electroplating, and if a difficult to solder material is first subjected to preplating with a material having good solderability, solder coating to a thickness of 10-50 micrometers can be carried out easily and with certainty by hot dipping.
The present inventors subsequently performed repeated investigations, and they found that by guaranteeing a thickness of 0.5‥5 micrometers as the thickness of base plating formed by electroplating, at the time of hot dipping, a plating thickness of 10-50 micrometers can be obtained easily and with certainty by carrying out hot dipping using existing equipment and under normal conditions, and they thereby completed the present invention.
Accordingly, the present invention is a solder coated material comprising a substrate comprising a difficult to solder material, an electroplated layer of a material having excellent solderability and a thickness of 0.5-5 micrometers which is provided on the substrate as base plating, and a hot dip solder plated layer having a thickness of 10-50 micrometers which is provided on the electroplated layer.
From a different aspect, the present invention is a manufacturing method for a solder coated material characterized by performing electroplating with a material having excellent solderability on necessary locations of a difficult to solder material, and then passing the difficult to solder material through a molten solder bath while applying ultrasonic waves if necessary and adhering molten solder to the electroplated locations.
In a preferred mode of the present invention, the molten solder bath may be maintained in an inert atmosphere, and the molten solder bath may be a wave soldering bath.
Furthermore, according to a different aspect, the present invention is a portion to be soldered of an electronic part having an electroplated layer of a material having excellent solderability with a thickness of 0.5-5 micrometers applied as base plating atop a substrate comprising a difficult to solder material and a hot dip plated layer of solder with a thickness of 10-50 micrometers applied atop the electroplate.
In the present invention, examples of materials to be soldered which can be used as the substrate include iron-nickel alloys.
Examples of the material having excellent solderability are any of gold, silver, copper, tin, nickel, and solder alloys, but preferably it is a solder alloy of a tin-silver alloy.
Examples of a portion to be soldered of an electronic part are lead frames of electronic parts, lids of packaged electronic parts, battery terminals, shields for modules, and connectors for surface mounted parts.